1. Field of the Invention
This invention relates to a microwave power limiter and, more particularly, to a power limiter including a dual gate field effect transistor.
2. Description of the Prior Art
Many commercial and military systems require a source of constant RF power while simultaneously tolerating a wide variation of RF input power to the same system. It is often a system requirement that this fixed output power level be obtained over a wide frequency bandwidth. Such a power limiter having these desirable characteristics may be used as an RF amplifier, front end receiver where fixed output power is necessary for radar terrain mapping and associated video osscilloscope displays. Other limiter applications include receivers that are used for phase and frequency identification for electronic countermeasure (ECM) systems. In this application, fixed input power is generally applied to phase discrimination for accurate conversion of frequency (or phase) into an output dc voltage. A limiter having these properties may also be utilized for bi-phase and digital communications systems where coded binary signals are received and cannot be reconstructed to constant power output levels independent of the amplitude variations of the receiving antenna.
One of the conventional approaches for achieving constant (or standardized) output power, independent of drive variation and frequency, is to utilize a multistage high gain amplifier chain. This amplifier chain is designed to constrain operation of several of the output stages to the overdriven or saturation region where the output power swing is greatly compressed in spite of large variations in the input power range to the chain. Both bipolar silicon transistors and tunnel diode amplifiers (TDA) are used in such multistage limiters. The bipolar transistor limiter is restricted, however, to low frequency applications and is not often selected for use. For other limiting amplifier circuits see U.S. Pat. No. 3,940,704 issued Feb. 24, 1976 and U.S. Pat. No. 3,999,084 issued Dec. 21, 1976 which use operational amplifiers and U.S. Pat. No. 3,932,768 issued Jan. 13, 1976 and U.S. Pat. No. 4,008,440 issued Feb. 15, 1977 which utilize differential amplifiers.
The TDA is in wide use and has heretofore provided desirable amplifier-limiter capabilities. The TDA limiter suffers, however, from diverse problems and disadvantages, limiting the applications and performance of the TDA limiter. For instance, the power output for a broadband TDA is limited to levels of about -6 dbm (0.25 mW) or less. This power output level produces an RMS output voltage of less than 0.1 volt, with a video detector circuit having an impedance of 100 ohms. Such output is too low to drive a conventional phase interferometer-discriminator and detector without the use of video amplifiers. Another disadvantage of the TDA is that since it is a two terminal device used as a reflection amplifier, circulators for each stage and isolators between every other stage are needed for stability of the limiter. Such a multiplicity of interconnected components typically using ferrite materials produces many reflections which add in-and-out-of-phase resulting in considerable fine and coarse grain structure which directly reduces the accuracy when used with a discriminator. In such a discriminator system, for example, a variation of about 0.1 db will case an attendant error of approximately 80 MHz in a 4GHz band system. Furthermore, the use of necessary circulators and isolators in a multistage TDA configuration requires extensive circuitry and thereby, a relatively costly device.
For microwave discriminator applications, the output power variation as the input to the discriminator circuit is to be minimal. For example, to approach a frequency accuracy of 1 MHz in a 4 GHz bandwidth, due to output power variations of the RF limited output wave, would require an output power deviation or flatness of 0.025%. This is equivalent to an incremental power output fluctuation 36 db from saturation over the total variation in input drive range. With an input drive range of, for instance, 30 db, the combined values of input power swing and output limiting is equivalent to an overall compression ratio of 66 db. Considering the capabilities and costs, such a performance would be difficult, if not impossible, to achieve with the TDA limiter.
Use of Gallium Arsenide (GaAs) single gate field effect transistors (FET) arranged in a multistage configuration and operated in a satured condition has been suggested. (See the article in Microwaves, entitled "GaAs FETs Gain Ground in Oscillators, Mixers and Limiters," June, 1977, pages 9-10, and a paper published in the minutes of the IEEE Microwave Theory and Techniques (MTT) conference in San Diego, June, 1977, entitled "A New Microwave Amplitude Limiter Using GaAs Field Effect Transistor," by S. Fukuda, M. Kitamura, Y. Ara and I. Haga.) The single gate FET has a relatively high parasitic capacitance and results in an undesirably high feedback capacitance between the gate and drain electrodes. This feedback capacitance tends to reduce the small signal gain of the amplifier. The RF feedthrough from input to output of the FET is also decreased by this capacitance resulting in reduced saturation properties. These deficiencies can often be overcome by increasing the number of FET stages in the limiter disadvantageously adding, however, to the cost as well as size of the device.